…#1742)

The two set() overloads in AnyServiceCallback and GenericServiceCallback were
selected by detail::can_be_nullptr, whose non-QNX specialization probed the raw
callable with decltype(std::declval<T&>() = nullptr). That expression is
ill-formed for some callables/compilers, disabling both overloads and breaking
compilation of create_service() (and the generic service) for affected
callbacks.

Replace the trait with a single set() that stores the callback and then checks
the resulting std::function for emptiness via detail::callback_is_null(), which
visits the variant. std::function's operator bool flags only genuine null
targets (e.g. a null function pointer); lambdas and bind expressions are never
empty, so behavior is preserved while the brittle SFINAE is removed. This also
collapses the duplicated overload pair in both headers.

Add regression tests for function-pointer and std::function callbacks (which
previously failed to compile) and null-rejection for both.

Fixes ros2#1742

Generated-by: Claude Opus 4.8 (Anthropic)
Signed-off-by: Pavel Guzenfeld <pavelguzenfeld@gmail.com>